Low-noise amplifiers (LNAs) are among the first signal processing components in a radio-frequency (RF) receiver chain. Typically, the target information-bearing signals arriving at the input of an LNA are weak and corrupted by noise. A well-designed LNA boosts the signal power of the incoming signal while minimizing the production of amplifier-induced artifacts, e.g., amplifier-generated noise and distortion, in the amplified signal. Thus, in addition to characteristics of any good signal amplifier, e.g., linear gain, stability and impedance-matched over the operating bandwidth, a good LNA must also have a low noise figure (NF) and high intermodulation and compression points.
The front-end of the receiver chain is often connected to an unbalanced transmission line on which a ground-referenced signal is delivered, which presents an interface problem in those modern RF receivers that implement differential signaling. Differential signaling, where the target signal's amplitude is the potential difference between two time-varying signal components, offers several advantages, not the least of which is cancellation of common mode noise. A common solution to adapting a single-ended signaling system, such as an unbalanced transmission line, to a receiver employing differential signaling is to install a balanced-unbalanced transformer, commonly referred to as a “balun” at or near the interface. However, this solution not only increases the receiver's size, complexity and cost, but conventional baluns are band-limited. Consequently, when the receiver is expected to accept signals that span a wide spectral region, conventional implementations incorporate multiple baluns, each to accommodate a sub-band of the target spectrum. Traditional television tuners, for example, operate in the very-high frequency (VHF) television broadcast band, which, in the US, spans the RF frequencies between 54 and 216 MHz and the ultra-high frequency (UHF) television broadcast band, which spans 470 MHz-806 MHz. The input circuitry in such television tuners is often composed of separate circuits for VHF and UHF bands, each with its own balun, LNA and, often downconverter.
Variable-gain LNAs (VG-LNAs) are often deployed where variation in incoming signal strength is expected. For broad dynamic range, a VG-LNA must provide not only amplification, but attenuation as well. Maintaining a wide operational frequency band in such a VG-LNA presents challenges in that parasitic loading of many attenuator circuits limit the spectral range of the amplifier.
Ongoing development efforts in radio front-end technology seek robust designs for broadband LNA circuits that can be situated at the single-ended to differential signaling interface with minimal size and cost.